In the present description, an oil well and a gas well are collectively referred to simply as “an oil well”. Accordingly, “a stainless steel for oil wells” as used herein includes a stainless steel for oil wells and a stainless steel for gas well. Also “a stainless steel pipe for oil wells” includes a stainless steel pipe for oil wells and a stainless steel pipe for gas well.
As used herein, the term “a high temperature” means, unless otherwise stated, a temperature not less than 150° C. Also as used herein, the symbol “%” relating to a chemical element means, unless otherwise stated, “mass %”.
A conventional oil well environment contains carbon dioxide gas (CO2) and/or chlorine ion (Cl−). For that reason, in a conventional oil well environment, a martensitic stainless steel containing 13% of Cr (hereafter, referred to as a “13% Cr steel”) is commonly used. The 13% Cr steel is excellent in carbonic-acid gas corrosion resistance.
Recently, the development of deep oil wells has advanced. A deep oil well has a high-temperature environment. Such high-temperature environment includes carbon dioxide gas or carbon dioxide gas and hydrogen sulfide gas. These gases are corrosive gases. Therefore, steel for oil wells to be used in deep oil wells is required to have a higher strength and a higher corrosion resistance than those of the 13% Cr steel.
The Cr content of a two-phase stainless steel is greater than that of the 13% Cr steel. Therefore, a two-phase stainless steel has a higher strength and a higher corrosion resistance than those of the 13% Cr steel. The two-phase stainless steel is, for example, a 22% Cr steel containing 22% of Cr, and a 25% Cr steel containing 25% of Cr. Although the two-phase stainless steel has a high strength and a high corrosion resistance, it includes many alloy elements, and therefore is expensive.
JP2002-4009A, JP2005-336595A, JP2006-16637A, JP2007-332442A, WO2010/050519, and WO2010/134498 propose stainless steels other than the above described two-phase stainless steel. The stainless steels disclosed in these literatures contain at the maximum 17 to 18.5% of Cr.
JP2002-4009A proposes a martensitic stainless steel for oil wells, which has a yield strength of not less than 860 MPa and a carbonic-acid gas corrosion resistance in a high-temperature environment. The chemical composition of the stainless steel disclosed in this literature contains 11.0 to 17.0% of Cr and 2.0 to 7.0% of Ni, and further satisfies: Cr+Mo+0.3 Si−40C−10N−Ni−0.3Mn≦10. The metal micro-structure of this stainless steel is predominantly made up of martensite, and contains not more than 10% of a retained austenite.
JP2005-336595A proposes a stainless steel pipe which has a high strength and carbonic-acid gas corrosion resistance in a high-temperature environment of 230° C. The chemical composition of the stainless steel pipe disclosed in this literature contains 15.5 to 18% of Cr, 1.5 to 5% of Ni, and 1 to 3.5% of Mo, satisfies Cr+0.65Ni+0.6Mo+0.55Cu−20C≧19.5 and also satisfies Cr+Mo+0.3Si−43.5C−0.4Mn−Ni−0.3Cu−9N≧11.5.
The metal micro-structure of this stainless steel pipe contains 10 to 60% of a ferrite phase, and not more than 30% of an austenite phase, the balance being a martensite phase.
JP 2006-16637A proposes a stainless steel pipe which has a high strength and carbonic-acid gas corrosion resistance in a high-temperature environment of more than 170° C. The chemical composition of the stainless steel pipe disclosed in this literature contains 15.5 to 18.5% of Cr, and 1.5 to 5% of Ni, satisfies Cr+0.65 Ni+0.6 Mo+0.55Cu−20C≧18.0 and also satisfies Cr+Mo+0.3Si−43.5C−0.4Mn−Ni−0.3Cu−9N≧11.5. The metal micro-structure of this stainless steel pipe may or may not include an austenite phase.
JP 2007-332442A proposes a stainless steel pipe which has a high strength of not less than 965 MPa, and a carbonic-acid gas corrosion resistance in a high-temperature environment exceeding 170° C. The chemical composition of the stainless steel pipe disclosed in this literature contains, by mass %, 14.0 to 18.0% of Cr, 5.0 to 8.0% of Ni, 1.5 to 3.5% of Mo, and 0.5 to 3.5% Cu, and satisfies Cr+2 Ni+1.1 Mo+0.7 Cu≦32.5. The metal micro-structure of this stainless steel pipe contains 3 to 15% of an austenite phase, the balance being a martensite phase.
WO2010/050519 proposes a stainless pipe which has a sufficient corrosion resistance even in a high-temperature carbon dioxide environment of 200° C., and further has a sufficient sulfide stress-corrosion cracking resistance even when the environment temperature of an oil well or gas well declines due to a temporary suspension of the collection of crude oil or gas. The chemical composition of the stainless steel pipe disclosed in this literature contains more than 16% and not more than 18% of Cr, more than 2% and not more than 3% of Mo, not less than 1% and not more than 3.5% of Cu, and not less than 3% and less than 5% of Ni, while Mn and N satisfy [Mn]×([N]−0.0045)≦0.001. The metal micro-structure of this stainless steel pipe contains 10 to 40% by volume fraction of a ferrite phase, and not more than 10% by volume fraction of a retained γ phase with a martensite phase being as the dominant phase.
WO2010/134498 proposes a high-strength stainless steel which has an excellent corrosion resistance in a high-temperature environment, and has an SSC resistance (sulfide stress-corrosion cracking resistance) at normal temperature. The chemical composition of the stainless steel disclosed in this literature contains more than 16% and not more than 18% of Cr, not less than 1.6% and not more than 4.0% of Mo, not less than 1.5% and not more than 3.0% of Cu, and more than 4.0% and not more than 5.6% of Ni, and satisfies Cr+Cu+Ni+Mo≧25.5 and −8≦30(C+N)+0.5Mn+Ni+Cu/2+8.2−1.1(Cr+Mo)≦−4. The metal micro-structure of this stainless steel contains a martensite phase, 10 to 40% of a ferrite phase, and a retained austenite phase, with a ferrite phase distribution rate being higher than 85%.